Yellow-green to yellow-emitting phosphors based on halogenated-aluminates

ABSTRACT

Disclosed herein are yellow-green and yellow-emitting aluminate based phosphors for use in white LEDs, general lighting, and LED and backlighting displays. In one embodiment of the present invention, the cerium-activated, yellow-green to yellow-emitting aluminate phosphor comprises the rare earth lutetium, at least one alkaline earth metal, aluminum, oxygen, at least one halogen, and at least one rare earth element other than lutetium, wherein the phosphor is configured to absorb excitation radiation having a wavelength ranging from about 380 nm to about 480 nm, and to emit light having a peak emission wavelength ranging from about 550 nm to about 600 nm.

CLAIM OF PRIORITY

This application claims the benefit of priority to U.S. ProvisionalApplication No. 61/450,310, filed Mar. 8, 2011, entitled PHOSPHORCOMPOSITION, by Yi-Qun Li et al., and is a continuation-in-part of U.S.patent application Ser. No. 13/181,226, filed Jul. 12, 2011, entitledGREEN-EMITTING, GARNET-BASED PHOSPHORS IN GENERAL AND BACKLIGHTINGAPPLICATIONS, by Yusong Wu et al., which claims the benefit of priorityto U.S. Provisional Application No. 61/364,321, filed Jul. 14, 2010,entitled GREEN-EMITTING, GARNET-BASED PHOSPHORS IN GENERAL ANDBACKLIGHTING APPLICATIONS, by Yusong Wu et al. and is acontinuation-in-part of U.S. patent application Ser. No. 11/975,356filed Oct. 18, 2007, entitled NANO-YAG:CE PHOSPHOR COMPOSITIONS ANDTHEIR METHODS OF PREPARATION, by Dejie Tao et al., which claims benefitof U.S. Provisional Application No. 60/853,382 filed Oct. 20, 2006,entitled NANO YAG:CE PHOSPHORS AND THE METHOD OF PREPARING THE SAME, byDejie Tao et al., which applications are incorporated herein byreference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present disclosure are directed in to yellow-green toyellow-emitting phosphors based on halogenated aluminates. Suchphosphors are applicable to a number of different technologic areas,including general lighting systems, white light illumination systemsbased on white LEDs, signal lights; indicator lights, etc., as well asdisplay applications such as display backlighting, plasma displaypanels, LED-based display panels, and the like.

2. Description of the Related Art

Embodiments of the present invention are directed to halogenatedaluminate-based phosphors that, when activated by cerium, and when dopedwith the rare earths lutetium and a second rare earth, which may begadolinium, emit visible light in the yellow-green to yellow portion ofthe electromagnetic spectrum. The phrase “visible light in theyellow-green to yellow portion of the electromagnetic spectrum” isdefined to mean light having a peak emission wavelength of about 550 nmto about 600 nm. Such phosphors may be used in commercial markets wherewhite light is generated using so-called “white light LEDs,” noting asan aside that this term is somewhat of a misnomer, since light emittingdiodes emit light of a specific monochromatic color and not acombination of wavelengths perceived as white by the human eye. The termis nonetheless entrenched in the lexicon of the lighting industry.

Historically, YAG:Ce (yittrium aluminate garnet activated with cerium)has been used to supply the yellow component of the light in thelighting systems mentioned above. In comparison to other phosphor hosts,particularly those based on the silicates, sulphates, nitridosilicates,and oxo-nitridosilicates, YAG:Ce has a relatively high absorptionefficiency when excited by blue light, is stable in high temperature andhumidity environments, and has a high quantum efficiency (QE>95%), allthe while displaying a broad emission spectrum.

One disadvantage to using a YAG:Ce based phosphor, other than inadequatecolor rendering in some situations, is that the peak emission of thisphosphor is too long, that is to say, too deep towards the orange or redfor use as a luminescent source in, for example, a backlightingapplication. An alternative to YAG:Ce is the cerium doped Lu₃Al₅O₁₂compound (LAG:Ce), which has the same crystalline structure as YAG:Ce, asimilar temperature and humidity stability as the yttrium-basedcompound, and likewise quantum efficiency. Despite these similarities,LAG:Ce exhibits a different peak emission wavelength than its YAGcounterpart; in the lutetium case, this peak wavelength is at about 540nm. This emission wavelength is still not short enough, however, to beideal for certain applications such as backlighting applications, andgeneral lighting applications, where appropriate.

Thus, what is needed in the art, particularly in fields related tobacklighting technologies and general lighting, is a phosphor with astructure comparable to a garnet in terms of temperature and humiditystability, but having at the same time a peak emission wavelengthranging from about 550 nm to about 600 nm.

SUMMARY OF THE INVENTION

Embodiments of the present disclosure are directed to yellow-green andyellow-emitting aluminate based phosphors for use in white LEDs, generallighting, and LED and backlighting displays.

In one embodiment of the present invention, the cerium-activated,yellow-green to yellow-emitting aluminate phosphor comprises the rareearth lutetium, at least one alkaline earth metal, aluminum, oxygen, atleast one halogen, and at least one rare earth element other thanlutetium, wherein the phosphor is configured to absorb excitationradiation having a wavelength ranging from about 380 nm to about 480 nm,and to emit light having a peak emission wavelength ranging from about550 nm to about 600 nm.

In another embodiment of the present invention, the yellow-green toyellow-emitting aluminate phosphor comprises a halogenated aluminate, anM²⁺X₂ additive, and a cerium activator; where M²⁺ is a divalent,alkaline earth metal selected from the group consisting of Mg, Sr, Ca,and Ba; X is a halogen selected from the group consisting of F, Cl, Br,and I; and wherein the M²⁺X₂ additive is included in the phosphor inamounts ranging up to about 5 wt %, the upper endpoint inclusive.

In another embodiment of the present invention, the yellow-green toyellow-emitting phosphor comprises a halogenated aluminate having theformula A₃B_(x)Al₅O₁₂C_(y):Ce³⁺, where A is at least one of Lu, La, Sc,Gd, or Tb; B is at least one of Mg, Sr, Ca, or Ba; C is at least one ofF, Cl, Br, or I; and y is about 2x, although y may be less than 2x byamounts (stoichiometrically) of up to 5, 10, 25, and 50 percent. Theyellow-green to yellow-emitting halogenated aluminate phosphor comprisesan aluminate configured to absorb excitation radiation having awavelength ranging from about 420 nm to about 480 nm, and to emit lighthaving a peak emission wavelength ranging from about 550 nm to about 600nm.

In another embodiment of the present invention, the yellow-green toyellow-emitting halogenated aluminate phosphor has the formula:(Lu_(1-x-y)A_(x)Ce_(y))₃B₂Al₅O₁₂C_(2z); where A is at least one of Sc,La, Gd, and Tb; B is at least one of Mg, Sr, Ca, and Ba; C is at leastone of F, C, Br, and I; 0≦x≦0.5; 0.001≦y≦0.2; and 0.001≦z≦0.5. In thisembodiment, A may be Gd; B may be Ba or Sr; and C may be F.

Other embodiments of the present invention are directed to generallighting or white LEDs, which comprise a radiation source configured toprovide radiation having a wavelength greater than about 280 nm; acerium-activated, yellow-green to yellow-emitting aluminate phosphorcomprising the rare earth lutetium, at least one alkaline earth metal,aluminum, oxygen, at least one halogen, and at least one rare earthelement other than lutetium, wherein the phosphor is configured toabsorb excitation radiation having a wavelength ranging from about 380nm to about 480 nm, and to emit light having a peak emission wavelengthranging from about 550 nm to about 600 nm. This embodiment includes atleast one of a red-emitting phosphor or a yellow-emitting phosphor.Alternative embodiments directed to general lighting or white LEDs maycomprise: a radiation source configured to provide radiation having awavelength greater than about 280 nm; a yellow-green to yellow-emittingphosphor comprising a halogenated aluminate having the formulaA₃B_(x)Al₅O₁₂C_(y):Ce³⁺, where A is at least one of Lu, La, Sc, Gd, orTb; B is at least one of Mg, Sr, Ca, or Ba; C is at least one of F, Cl,Br, or I; where y is about equal to or less than 2x; and at least one ofa red-emitting phosphor or a yellow-emitting phosphor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the SEM morphology of Lu_(2.91)Ce_(0.09)Al₅O₁₂ withdifferent MgF₂ additive concentrations, illustrating that particle sizesbecome larger and more homogeneous as the amount of the MgF₂ additive isincreased;

FIG. 2 is a series of x-ray diffraction (XRD) patterns of exemplaryY_(2.91)Ce_(0.09)Al₅O₁₂ phosphors with different MgF₂ additiveconcentrations;

FIG. 3 is a series x-ray diffraction (XRD) patterns of exemplaryLu_(2.91)Ce_(0.09)Al₅O₁₂ phosphors with different MgF₂ additiveconcentrations;

FIG. 4 is a series of the x-ray diffraction (XRD) patterns of exemplaryLu_(2.91)Ce_(0.09)Al₅O₁₂ phosphors having a 5 wt % MgF₂ additive and a 5wt % SrF₂ additive;

FIG. 5 is the emission spectra of a series of exemplaryY_(2.91)Ce_(0.09)Al₅O₁₂ phosphors with different levels of MgF₂additive, the emission spectra obtained by exciting the phosphors with ablue LED;

FIG. 6 is the normalized emission spectra of a series of exemplaryLu_(2.91)Ce_(0.09)Al₅O₁₂ phosphors with different MgF₂ additiveconcentrations under blue LED excitation;

FIG. 7 is the emission spectra of Lu_(2.91)Ce_(0.09)Al₅O₁₂ phosphorswith different MgF₂ additive under blue LED excitation;

FIG. 8 is the normalized emission spectra of Lu_(2.91)Ce_(0.09)Al₅O₁₂phosphors with different MgF₂ additive under blue LED excitation; theresults show that the emission peak of Lu_(2.91)Ce_(0.09)Al₅O₁₂ shiftsto short wavelength with certain amount of MgF₂ additive, and that thegreater the amount of the MgF₂ additive, the shorter emission peakwavelength;

FIG. 9 is a normalized emission spectra of a Lu_(2.91)Ce_(0.09)Al₅O₁₂phosphor with 5 wt % MgF₂ and 5 wt % SrF₂ additives where the phosphorhas been excited with a blue LED; the results are compared with acontrol sample that contains no halogenated salts as an additive; theresults illustrate that the emission peak shifts to shorter wavelengthswith the MgF₂ synthesized compound than it does for the SrF₂ synthesizedcompound;

FIG. 10 shows how the emission wavelength of a series of exemplaryLu_(2.91)Ce_(0.09)Al₅O₁₂ phosphors decreases as the concentration of anSrF₂ additive is increased;

FIG. 11 is the normalized excitation spectra of a series of exemplaryLu_(2.91)Ce_(0.09)Al₅O₁₂ phosphors with different MgF₂ additiveconcentrations, showing that the excitation spectra becomes more narrowwhen the MgF₂ additive concentration is increased;

FIG. 12 shows the temperature dependence of an exemplaryLu_(2.91)Ce_(0.09)Al₅O₁₂ phosphor with a 5wt % MgF₂ additive;

FIG. 13 shows the spectra of a white LED that includes an exemplarygreen-emitting, aluminate-based phosphor having the formulaLu_(2.91)Ce_(0.09)Al₅O₁₂ with 5 wt % SrF₂ additive; the white LED alsoincludes a red phosphor having the formula(Ca_(0.2)Sr_(0.8))AlSiN₃:Eu²⁺, and when both green and red phosphors areexcited with an InGaN LED emitting blue light, the resulting white lighthad the color properties CIE x=0.24, and CIE y=0.20.

FIG. 14 is the spectra of a white LED with the following components: ablue InGaN LED, a green garnet having the formulaLu_(2.91)Ce_(0.09)Al₅O₁₂ with either 3 or 5 wt % additives, a rednitride having the formula (Ca_(0.2)Sr_(0.8))AlSiN₃:Eu²⁺ or a silicatehaving the formula (Sr_(0.5)Ba_(0.5))₂SiO₄:Eu²⁺, wherein the white lighthas the color coordinates CIE (x=0.3, y=0.3).

FIG. 15 is the spectra of the white LED systems of FIG. 14, in thisinstance measured at 3,000 K.

FIG. 16 is a table giving exemplary phosphors according to the generalformula (Lu_(1-x-y)A_(x)Ce_(y))₃B_(z)Al₅O₁₂C_(2z), where A is Gd, B iseither Ba or Sr, and C is F; the table also lists comparative CIE x andy coordinates of the emitted light; the peak emission wavelength in nm,relative intensities, and D50 particle size in microns.

FIGS. 17A-B shows that the peak emission wavelength of these halogenatedaluminates ranged overall from about 550 nm to about 580 nm as the Gdlevel was increased, where the Ba level was fixed stoichiometrically at0.15 for the Ba series, and where the Sr level was fixedstoichiometrically at 0.34; and

FIGS. 18A-B are the x-ray diffraction patterns of both the Ba series andthe Sr series of phosphors whose luminosity data was depicted in FIGS.17A-B.

DETAILED DESCRIPTION OF THE INVENTION

A yttrium aluminum garnet compound activated with the rare earth cerium(YAG:Ce) has been, historically, one of the most common choices ofphosphor material made if the desired application was either high powerLED lighting, or cool white lighting of a non-specific, general nature.As one might expect, there is a requirement in general lighting forhighly efficient components, both in the case of the LED chip supplyingthe blue light component of the resultant white light, and theexcitation radiation for the phosphor, where the phosphor typicallysupplies the yellow/green constituent of the resulting product whitelight.

As discussed in the previous section of this disclosure, YAG:Cedemonstrates this desired high efficiency, having a quantum efficiencygreater than about 95 percent, and it would therefore appear to be adifficult task to improve upon this number. But it is known in the artthat the efficiency of an LED chip increases with a decrease in emissionwavelength, and thus it would appear, in theory anyway, that theefficiency of a general lighting system will be enhanced if a phosphorpaired with an LED chip emitting at shorter wavelengths may be excitedby those shorter wavelengths. The problem with this strategy,unfortunately, is that the emission efficiency of a YAG:Ce phosphordecreases when the wavelength of its blue excitation radiation isreduced to a level below about 460 nm.

The repercussions of this are, of course, that YAG:Ce should really onlybe paired with an LED chip having an emission wavelength no less thanabout 450 to 460 nm. But it is also known in the art that photonenergies of the phosphor's excitation radiation depend strongly on thestructure of the anionic polyhedron (comprising oxygen atoms in thiscase) surrounding the activator cation (cerium). It follows that theefficiency of the system may be enhanced if the excitation range of agarnet-based phosphor might be extended towards shorter wavelengthsrelative to a YAG:Ce phosphor. Thus, one of the objects of the presentinvention include altering the structure and nature of this anionicpolyhedron to shift the excitation range the phosphor “desires” to seeto shorter wavelengths relative to that of the traditional YAG:Ce, whilemaintaining in the meantime (or even improving) the enhanced propertiesthat many garnets display.

The present disclosure will be divided into the following sections:first, a chemical description (using stoichiometric formulas) of thepresent halogenated aluminates will be given, followed by a briefdescription of viable synthetic methods that may be used to producethem. The structure of the present halogenated aluminates will bediscussed next, along with its relationship to experimental datacomprising wavelength and photoluminescent changes upon the inclusion ofcertain halogen dopants. Finally, the role these yellow-green andyellow-emitting phosphors may play in white light illumination, generallighting, and backlighting applications will be presented with exemplarydata.

Chemical Description of the Present Halogenated Aluminate-BasedPhosphors

The yellow to green-emitting, aluminate-based phosphors of the presentinvention contain both alkaline earth and halogen constituents. Thesedopants are used to achieve the desired photoemission intensity andspectral properties, but the fact that simultaneous alkaline earth andhalogen substitutions provide a sort of self-contained charge balance isfortuitous as well. Additionally, there may be other advantageouscompensations having to do with the overall changes to the size of theunit cell: while substitutions of any of Sc, La, Gd, and/or Tb for Lu(either individually, or in combinations) may tend to expand or contractthe size of the cell, the opposite effect may occur with substitutionsof halogen for oxygen.

There are several ways to describe the formula of the present phosphors.In one embodiment, a green emitting, cerium-doped, aluminate-basedphosphor may be described by the formula(Lu_(1-a-b-c)Y_(a)Tb_(b)A_(c))₃(Al_(1-d)B_(d))₅(O_(1-e)C_(e))₁₂:Ce,Eu,where A is selected from the group consisting of Mg, Sr, Ca, and Ba; Bis selected from the group consisting of Ga and In; C is selected fromthe group consisting of F, Cl, and Br; 0≦a≦1; 0≦b≦1; 0≦c≦0.5; 0≦d≦1; and0≦e≦0.2. The “A” element, which may be any of the alkaline earthelements Mg, Sr, Ca, and Ba, used either solely or in combination, isvery effective in shifting emission wavelength to shorter values. Thesecompounds will be referred to in the present disclosure as “halogenatedLAG-based” aluminates, or simply “halogenated aluminates.”

In an alternative embodiment, the present yellow to green-emitting,aluminate-based phosphors may be described by the formula(Y,A)₃(Al,B)₅(O,C)₁₂:Ce³⁺, where A is at least one of Tb, Gd, Sm, La,Lu, Sr, Ca, and Mg, including combinations of those elements, whereinthe amount of substitution of those elements for Y ranges from about 0.1to about 100 percent in a stoichiometric manner. B is at least one ofSi, Ge, B, P, and Ga, including combinations, and these elementssubstitute for Al in amounts ranging from about 0.1 to about 100 percentstoichiometrically. C is at least one of F, Cl, N, and S, includingcombinations, substituting for oxygen in amounts ranging from about 0.1to about 100 percent stoichiometrically.

In an alternative embodiment, the present yellow to green-emitting,aluminate-based phosphors may be described by the formula(Y_(1-x)Ba_(x))₃Al₅(O_(1-y)C_(y))₁₂:Ce³⁺, where x and y each range fromabout 0.001 to about 0.2.

In an alternative embodiment, a yellow-green to green-emitting,aluminate-based phosphor may be described by the formula (A_(1-x)³⁺B_(x) ²⁺)_(m)Al₅(O_(1-y) ²⁻C_(y) ¹⁻)_(n):Ce³⁺, where A is selectedfrom the group consisting of Y, Sc, Gd, Tb, and Lu; B is selected fromthe group consisting of Mg, Sr, Ca, and Ba; C is selected from the groupconsisting of F, Cl, and Br; 0≦x≦0.5; 0≦y≦0.5; 2≦m≦4; and 10≦n≦14.

In an alternative embodiment, a yellow-green to green-emitting,aluminate-based phosphor may be described by the formula (A_(1-x)³⁺B_(x) ²⁺)_(m)Al₅(O_(1-y) ²⁻C_(y) ¹⁻)_(n):Ce³⁺, where A is selectedfrom the group consisting of Y, Sc, Gd, Tb, and Lu; B is selected fromthe group consisting of Mg, Sr, Ca, and Ba; C is selected from the groupconsisting of F, Cl, and Br; 0≦x≦0.5; 0≦y≦0.5; 2≦m≦4; and 10≦n≦14;subject to the proviso that m is not equal to 3.

In an alternative embodiment, a yellow-green to green-emitting,aluminate-based phosphor may be described by the formula (A_(1-x)³⁺B_(x) ²⁺)_(m)Al₅(O_(1-y) ²⁻C_(y) ¹⁻⁾ _(n):Ce³⁺, where A is selectedfrom the group consisting of Y, Sc, Gd, Tb, and Lu; B is selected fromthe group consisting of Mg, Sr, Ca, and Ba; C is selected from the groupconsisting of F, Cl, and Br; 0≦x≦0.5; 0≦y≦0.5; 2≦m≦4; and 10≦n≦14;subject to the proviso that n is not equal to 12.

In an alternative embodiment, a yellow to green-emitting,aluminate-based phosphor may be described by the formula(Lu_(1-x-y)A_(x)Ce_(y))₃B_(z)Al₅O₁₂C_(2z), where A is at least one ofSc, La, Gd, and Tb; B is at least one of the alkaline earths Mg, Sr, Ca,and Ba; C is at least one of the halogen elements F, C, Br, and I; andthe values of the parameters x, y, z are 0≦x≦0.5; 0.001≦y≦0.2; and0.001≦z≦0.5. It is noted that “at least one” of with regard to theformulas in this disclosure means that the elements in that group mayappear in the phosphor either individually, or in combinations, whereany combinations of any of the elements in that group are allowable,provided that the total amount of that group satisfies the rule assignedto it in terms of overall stoichiometric amounts.

One of ordinary skill in the art will appreciate that the relationshipbetween the amounts of C, the halogen, and B, the alkaline earth, maynot always be present in the phosphor product at the expected ratio of2:1 (stoichiometrically speaking) after a processing step such assintering if the C and B components are added to the starting mix ofmaterials in the form of an alkaline earth salt (e.g., B²⁺C₂). This isbecause the halogen component is known to be volatile, and in someinstances, some of the C is lost relative to B such that the ratio of Bto C in the final phosphor product is less than 2:1. Thus, in analternative embodiment of the present invention, the amount of C is lessthan 2z in the formula of paragraph [0045] by an amount of up to 5percent by number. In various other embodiments, the amount of C is lessthan 2z by an amount of up to 10, 25, and 50 percent stoichiometrically.

Synthesis

Any number of methods may be used to synthesize the present yellow-greento yellow-emitting, aluminate-based phosphors, methods that may involveboth solid state reaction mechanisms as well as liquid mixingtechniques. Liquid mixing includes such methods as co-precipitation andsol-gel techniques.

One embodiment of preparation involves a solid state reaction mechanismcomprising the steps:

-   -   (a) desired amounts of the starting materials CeO₂, Y₂O₃,        lutetium salts including the nitrates, carbonates, halides,        and/or oxides of lutetium, salts of the other rare earths Sc,        La, Gd, and Tb, and M²⁺X₂, where M is a divalent alkaline earth        metal selected from the group consisting of Mg, Sr, Ca, and Ba,        and X is a halogen selected from the group consisting of F, Cl,        Br, and I were combined to form a mixture of starting powders;    -   (b) the mix of starting powders from step (a) is dry-mixed using        any conventional method, such as ball milling, and typical        mixing times using ball milling are greater than about 2 hours        (in one embodiment about 8 hours);    -   (c) sintering the mixed starting powders from step (b) at a        temperature of about 1400° C. to about 1600° C. for about 6 to        about 12 hours in a reducing atmosphere (the purpose of this        atmosphere is for a reduction of the ammonia-based compounds);    -   (d) crushing the sintered product from step (c), and washing it        with water; and    -   (e) drying the washed product from step (d), wherein the drying        conditions may be constitute a time of about 12 hours at a        temperature of about 150° C.

The present aluminates may be synthesized by liquid mixing techniques.An example of the synthesis of a non-halogenated LAG compound having theformula Lu_(2.985)Ce_(0.015)Al₅O₁₂ using co-precipitation has beendescribed by H.-L. Li et al. in an article titled “Fabrication ofTransparent Cerium-Doped Lutetium Aluminum Garnet Ceramics byCo-Precipitation Routes,” J. Am. Ceram. Soc. 89 [7] 2356-2358 (2006).These non-halogenated LAG compounds contained no alkaline earthconstituents. The article is incorporated herein in its entirety, as itis contemplated that a similar co-precipitation method may be used toproduce the halogenated LAGs of the present disclosure with alkalineearth constituents.

An example of the synthesis of a halogenated YAG compound using asol-gel technique has been described in U.S. Pat. No. 6,013,199 by E.McFarland et al., to Symyx Technologies, titled “Phosphor materials.”These (possibly) halogenated YAG compounds contained no alkaline earthconstituents. This patent is incorporated herein in its entirety, as itis contemplated that a similar sol-gel method may be used to produce thehalogenated YAG compounds of the present disclosure with alkaline earthconstituents.

FIG. 1 shows the SEM morphology of an exemplary Lu_(2.91)Ce_(0.09)Al₅O₁₂phosphors with different MgF₂ additive concentrations, synthesized viathe solid state mechanisms described above. The morphology as revealedby scanning electron microscope (SEM) shows that particle sizes becomelarger, and more homogeneous, as the amount of the MgF₂ additive isincreased.

Crystal Structure of the Present Yellow-Green to Yellow EmittingAluminates

The crystal structure of the present yellow-green to yellow aluminatesis similar to that of the yttrium aluminum garnet, Y₃Al₅O₁₂, and inkeeping with this well studied YAG compound, the present aluminates maybelong to the space group Ia3d (no. 230). This space group, as itpertains to YAG, has been discussed by Y. Kuru et al. in an articletitled “Yttrium Aluminum Garnet as a Scavenger for Ca and Si,” J. Am.Ceram. Soc. 91 [11] 3663-3667 (2008). As described by Y. Kuru et al.,YAG has a complex crystal consisting of 160 atoms (8 formula units) perunit cell, where the Y³⁺ occupy positions of multiplicity 24, Wyckoffletter “c,” and site symmetry 2.22, and the O²⁻ atoms occupy positionsof multiplicity 96, Wyckoff letter “h,” and site symmetry 1. Two of theAl³⁺ ions are situated on octahedral 16(a) positions, whereas theremaining three Al³⁺ ions are positioned on tetrahedral 24(d) sites.

The lattice parameters of the YAG unit cell are a=b=c=1.2008 nm, andα=β=γ=90°. Whereas substitution of lutetium for yttrium is expected toexpand the size of the unit cell, the angles between the unit cell axesare not expected to change, and the material will retain its cubiccharacter.

FIG. 2 shows the x-ray diffraction (XRD) patterns of a series ofexemplary Y_(2.91)Ce_(0.09)Al₅O₁₂ phosphors with different MgF₂ additiveconcentrations, showing how the addition of an alkaline earth and ahalogen (MgF₂) component shifts high angle diffraction peaks to highervalues of 2θ. This means that the lattice constants become smallerrelative to a YAG component with no alkaline earth/halogen, and furtherindicates that Mg²⁺ is being incorporated into the crystal lattice,occupying Y³⁺ positions.

FIG. 3 shows the x-ray diffraction (XRD) pattern of a series ofexemplary phosphors in an analogous manner to FIG. 2, except that thistime the series of compounds are Lu_(2.91)Ce_(0.09)Al₅O₁₂ phosphors withdifferent MgF₂ additive concentrations, where lutetium-based compoundsare being studied, rather than yttrium-based compounds.

FIG. 4 shows the x-ray diffraction (XRD) pattern of a series ofexemplary Lu_(2.91)Ce_(0.09)Al₅O₁₂ phosphors having either a 5 wt % MgF₂and 5 wt% SrF₂ additive: this experiment shows a comparison of the Mgconstituent versus an Sr constituent. The data shows that with the MgF₂additive in the Lu_(2.91)Ce_(0.09)Al₅O₁₂ lattice, high angle diffractionpeak move to greater values of 2θ, meaning that lattice constants becomesmaller. Alternatively, with SrF₂ additive, high angle diffraction peaksmove to smaller values of 2θ, meaning that the lattice constantsincrease. It will be apparent to those skilled in the art that both Mg²⁺and Sr²⁺ are being incorporated into the Lu_(2.91)Ce_(0.09)Al₅O₁₂lattice and occupying Lu³⁺ positions. These peak shifts in positionoccur because Mg²⁺, with its ionic radius of 0.72 Π, is smaller thanLu³⁺ (0.86 Π), while Sr²⁺ (1.18 Π) is bigger than Lu³⁺.

Mechanism of Alkaline Earth and Halogen Influence on Optical Properties

In one embodiment of the present invention, Ce³⁺ is the luminescentactivator in the aluminate-based phosphor. The transition between the 4fand 5d energy levels of the Ce³⁺ ion corresponds to excitation of thephosphor with blue light; green light emission from the phosphor is aresult from the same electronic transition. In the aluminate structure,the Ce³⁺ is located at the center of an octahedral site formed by apolyanionic structure of six oxygen ions. It will be appreciated bythose skilled in the art that according to crystal field theory, thesurrounding anions (which may also be described as ligands) induce anelectrostatic potential on the 5d electron of the central cation. The 5denergy level splitting is 10Dq, where Dq is known to depend on theparticular ligand species. From the spectrochemical series it may beseen that the Dq of a halide is smaller than that of oxygen, and thus itfollows that when oxygen ions are replaced by halide ions, the Dq willdecrease correspondingly.

The implications of this are that the band gap energy; that is to say,the energy difference between the 4f and 5d electronic levels, willincrease with substitution of oxygen ions with halide in the polyanioniccages surrounding activator ions. This is why the emission peak shiftsto shorter wavelength with halogen substitution. Simultaneously, withthe introduction of halide ions in the oxygen polyanionic structuresforming octahedral sites, a corresponding cation may also replace aportion of the Lu (and/or Sc, La, Gd, and Tb) content. If the cationreplacing Lu (and/or the other rare earths) is a smaller cation, theresult will be a shift of the emission peak towards the blue end of thespectrum. The emitted luminescence will have a shorter wavelength thanotherwise would have occurred. In contrast, if the cation replacing Luis a larger cation, such as Sr or Ba, the result will be a shift of theemission peak towards the red end of the spectrum. In this case, theemitted luminescence will have a longer wavelength.

Combined with the effects of the halide, Mg as an alkaline earthsubstituent will be a better choice than Sr if a blue-shift is desired,and this will be shown experimentally in the following portions of thepresent disclosure. It is also known the LAG emission peak is a doubletdue to spin-orbit coupling. As the blue-shift occurs, the emission withshorter wavelength is biased and its intensity increasescorrespondingly. This trend is not only helpful to the blue-shift of theemission, but also enhances photoluminescence.

FIG. 5 is the emission spectra of a series of exemplaryY_(2.91)Ce_(0.09)Al₅O₁₂ phosphors with different levels of MgF₂additive, the emission spectra obtained by exciting the phosphors with ablue LED. This data shows that with increasing amounts of MgF2 thephotoluminescent intensity increases and the peak emission wavelengthshifts to shorter values. Though not shown on FIG. 5, the presentinventors have data for a 5 wt % addition of BaF₂ to the startingpowders: this phosphor showed a significant increase in photoluminescentintensity relative to the three magnesium-containing phosphors, and apeak emission wavelength that the same about as that of the 1 wt %sample.

A normalized version of the data from FIG. 5 is shown in FIG. 6. FIG. 6is the normalized emission spectra of the same series of exemplaryLu_(2.91)Ce_(0.09)Al₅O₁₂ phosphors with different MgF₂ additiveconcentrations under blue LED excitation, but where normalizing thephotoluminescent intensity to a single value highlight that the emissionpeak of Y_(2.91)Ce_(0.09)Al₅O₁₂ shifts to short wavelength withincreasing amounts of the MgF₂ additive. The greater the amount of theMgF₂ additive, the shorter emission peak wavelength. This is the sametrend with demonstrated by a Lu_(2.91)Ce_(0.09)Al₅O₁₂ phosphor, as willbe demonstrated next.

FIG. 7 is the emission spectra of a series of exemplaryLu_(2.91)Ce_(0.09)Al₅O₁₂ phosphors with different levels of MgF₂additive, the emission spectra obtained by exciting the phosphor with ablue LED. This data is analogous to that of FIG. 5, except thatlutetium-based rather than yttrium-based compounds are being studied. Aswith the yttrium data, this data for lutetium shows similar trends forthe shift in emission wavelength, though those trends forphotoluminescent intensity are not, perhaps, as similar.

The Lu_(2.91)Ce_(0.09)Al₅O₁₂ emission spectra of FIG. 7 has beennormalized to emphasize the effect of the addition of halogen salt onpeak emission wavelength; the normalized version of the data is shown inFIG. 8. As in the yttrium case, peak emission shifts to shorterwavelength with increasing amounts of MgF₂ additive; that is to say, thegreater the amount of the MgF₂ additive, the shorter emission peakwavelength. The amount of wavelength shift upon increasing the amount ofthe MgF2 additive from zero (no additive) to about 5 wt % of theadditive was observed to be about 40 nm; from about 550 nm to about 510nm.

Each of the graphs in FIGS. 5-8 have plotted their respective spectra asa series of phosphor compositions with increasing additiveconcentration, starting at no additive, and ending with the highestconcentration of the series at 5 wt %. To emphasize a comparison of theSrF₂ additive with the MgF₂ additive; in other words, a phosphor with anSr alkaline earth and fluorine content with a phosphor having a Mgalkaline earth and fluorine content, the phosphors have been plottedtogether in FIG. 9: a phosphor with no additive, a phosphor with 5 wt %SrF₂, and a phosphor with 5 wt % MgF₂. The phosphor is based on thesample Lu_(2.91)Ce_(0.09)Al₅O₁₂.

The emission spectra data in FIG. 9 has been normalized to betteremphasize the effects on optical properties rendered by the inclusionthe halogen and alkaline earths. When excited with a blue LED, theresult illustrates that the emission peak shift to shorter wavelengthswith the addition of MgF₂ and SrF₂. The Lu_(2.91)Ce_(0.09)Al₅O₁₂ samplewith no additive shows a peak emission wavelength at about 550 nm; witha 5 wt % SrF₂ additive the peak emission wavelength shifts to about 535nm, and with a 5 wt % MgF₂ additive the wavelength shifts even furtherto about 510 nm.

FIG. 10 shows how the emission wavelength of a series of exemplaryLu_(2.91)Ce_(0.09)Al₅O₁₂ phosphors decreases as the concentration of anSrF₂ additive is increased. Peak emission wavelength has been plotted asa function of the amount of the SrF₂ additive; samples having an SrF₂additive content of 1, 2, 3, and 5 wt % were tested. The results showthat the peak emission wavelength was about the same for the 1 and 2 wt% samples, the wavelength being about 535 nm; as the SrF₂ additive isincreased to 3 wt % the peak emission wavelength decreases to about 533nm. With a further increase of SrF₂ additive to 5 wt % peak wavelengthdrops precipitously to about 524 nm.

Excitation Spectra and Temperature Dependence

FIG. 11 is the normalized excitation spectra of a series of exemplaryLu_(2.91)Ce_(0.09)Al₅O₁₂ phosphors with different MgF₂ additiveconcentrations, showing that the excitation spectra becomes more narrowwhen the MgF₂ additive concentration is increased. The data shows thatthe present green emitting, aluminate-based phosphors exhibit a wideband of wavelengths over which the phosphors may be excited, rangingfrom about 380 to about 480 nm.

The thermal stability of the present garnet phosphors is exemplified bythe lutetium containing compound Lu_(2.91)Ce_(0.09)Al₅O₁₂ with a 5wt %MgF₂ additive; its thermal stability is compared with the commerciallyavailable phosphor Ce³⁺:Y₃Al₅O₁₂ in FIG. 12. It may be observed that thethermal stability of the Lu_(2.91)Ce_(0.09)Al₅O₁₂ compound is evenbetter than the YAG.

Applications to Backlighting and White Light Illumination Systems

According to further embodiments of the present invention, the presentgreen emitting, aluminate-based phosphors may be used in white lightillumination systems, commonly known as “white LEDs,” and inbacklighting configurations for display applications. Such white lightillumination systems comprise a radiation source configured to emitradiation having a wavelength greater than about 280 nm; and a halideanion-doped green aluminate phosphor configured to absorb at least aportion of the radiation from the radiation source, and emit lighthaving a peak wavelength ranging from 480 nm to about 650 nm.

FIG. 13 shows the spectra of a white LED that includes an exemplarygreen-emitting, aluminate-based phosphor having the formulaLu_(2.91)Ce_(0.09)Al₅O₁₂ with a 5 wt % SrF₂ additive. This white LEDfurther includes a red phosphor having the formula(Ca_(0.2)Sr_(0.8))AlSiN₃:Eu²⁺. When both green aluminate and red nitridephosphors are excited with an InGaN LED emitting blue light, theresulting white light displayed the color coordinates CIE x=0.24, andCIE y=0.20.

FIG. 14 is the spectra of a white LED with the following components: ablue InGaN LED, a green garnet having the formulaLu_(2.91)Ce_(0.09)Al₅O₁₂ with either 3 or 5 wt % additives, a rednitride having the formula (Ca_(0.2)Sr_(0.8))AlSiN₃:Eu²⁺ or a silicatehaving the formula (Sr_(0.5)Ba_(0.5))₂SiO₄:Eu²⁺, wherein the white lighthas the color coordinates CIE (x=0.3, y=0.3). The sample that shows themost prominent double peak is the one labeled “EG3261+R640,” where theEG3261 designation represents the (Sr_(0.5)Ba_(0.5))₂SiO₄:Eu²⁺ phosphor,in combination with the red R640 (Ca_(0.2)Sr_(0.8))AlSiN₃:Eu²⁺ phosphoremitting at about 640 nm. The two peaks labeled LAG (3 wt % MgF₂)+R640and LAG (5 wt % SrF₂)+R640 demonstrate a much more uniform emission ofperceived white light over the wavelength range 500 to 650 nm, anattribute desirable in the art.

FIG. 15 is the spectra of the white LED systems of FIG. 14, in thisinstance measured at 3,000 K.

In embodiments of the present invention, the red nitride that may beused in conjunction with the green aluminate may have the generalformula (Ca,Sr)AlSiN₃:Eu²⁺, where the red nitride may further comprisean optional halogen, and wherein the oxygen impurity content of the rednitride phosphor may be less than equal to about 2 percent by weight.The yellow-green silicates may have the general formula(Mg,Sr,Ca,Ba)₂SiO₄:Eu²⁺, where the alkaline earths may appear in thecompound either individually, or in any combination, and wherein thephosphor may be halogenated by F, Cl, Br, or I (again, eitherindividually, or in any combination).

Optical and Physical Data in Table Form

A summary of exemplary data is tabulated in the two tables at the end ofthis specification. In Table 1 is the testing results of aLu_(2.91)Ce_(0.09)Al₅O₁₂ based phosphor with three different MgF₂additive levels. Table 2 tabulates the testing results of aLu_(2.91)Ce_(0.09)Al₅O₁₂ based compound with four different SrF₂additive. These results summarize and confirm that MgF₂ and SrF₂additives in Lu_(2.91)Ce_(0.09)Al₅O₁₂ shift the emission peak wavelengthto shorter wavelengths, where the emission intensity is increased withincreasing MgF₂ and SrF₂ concentration. The particle size also increaseswith the increasing MgF₂ and SrF₂ additive concentration.

Yellow-green to Yellow Emitting, Rare Earth Doped Aluminate-BasedPhosphors

The rare earth doping of a specific series of yellow-green to yellowemitting, halogenated aluminates were tested by the present inventors,where the phosphors had the general formula(Lu_(1-x-y)A_(x)Ce_(y))₃B_(z)Al₅O₁₂C_(2z). As disclosed above, A is atleast one of Sc, La, Gd, and Tb; B is at least one of the alkalineearths Mg, Sr, Ca, and Ba; C is at least one of the halogen elements F,C, Br, and I; and the values of the parameters x, y, z are 0≦x≦0.5;0.001≦y≦0.2; and 0.001≦z≦0.5. In this series of phosphors, the rareearth dopant was Gd, and the alkaline earth was either Ba or Sr. Thehalogen was F in all of the compounds tested in this series ofexperiments. The formulas of the specific aluminates tested are shown inFIG. 16.

For the purpose of this disclosure, a green emission will be defined ashaving a peak emission wavelength of from about 500 nm to about 550 nm.Emissions extending from about 550 nm to about 600 nm may be describedas containing wavelengths that change from a yellow-green color to ayellow color. The addition of Gd doping converts the phosphor from asubstantially green-emitting sample to a substantially yellow sample inthe experiments described; though not shown, increasing the Gdconcentration even further (from about 0.33 for Ba samples and fromabout 0.13 for Sr samples) shifts the emission further towards and intothe yellow region of the electromagnetic spectrum. Makinggeneralizations can be difficult because the peak emission wavelengthdepends not only on the choice and level of the rare earth(s) dopantspresent in addition to lutetium (e.g., Gd in addition to Lu), but alsoon the selection and amounts of the included alkaline earth(s) and thehalogen(s). The halogenated aluminates in the present disclosure aredefined to emit in the yellow-green to the yellow region of theelectromagnetic spectrum, at wavelengths from about 550 nm to about 600nm. Green-emitting halogenated aluminates emit at peak wavelengthsranging substantially from about 500 nm to about 550 nm. Forgreen-emitting aluminates, see U.S. patent application Ser. No.13/181,226 filed Jul. 12, 2011, assigned to the same assignee as thepresent application, and hereby incorporated herein in its entirety.

The data in FIGS. 16 and 17A-B shows that the peak emission wavelengthof these halogenated aluminates ranged overall from about 550 nm toabout 580 nm as the Gd level was increased, where the Ba level was fixedstoichiometrically at 0.15 for the Ba series, and where the Sr level wasfixed stoichiometrically at 0.34 for the Sr series (the concentration isstoichiometric, meaning by number, not by weight). The Ce activatorlevel was also fixed stoichiometrically at 0.03 for all of the samples.Specifically, for the Ba samples, the peak emission wavelength increasedfrom 554 nm to 565 nm to 576 nm as the Gd amount was increasedstoichiometrically from 0.07 to 0.17 to 0.33, respectively. For the Srsamples, the peak emission wavelength increased from 551 nm to 555 nm to558 nm as the Gd amount was increased stoichiometrically from 0.03 to0.07 to 0.13, respectively.

The actual compounds in the Ba series were, respectively,(Lu_(0.90)Gd_(0.07)Ce_(0.03))₃Ba_(0.15)Al₅O₁₂F_(0.30),(Lu_(0.80)Gd_(0.17)Ce_(0.03))₃Ba_(0.15)Al₅O₁₂F_(0.30), and(Lu_(0.64)Gd_(0.33)Ce_(0.03))₃Ba_(0.15)Al₅O₁₂F_(0.30). The actualcompounds tested in the Sr series were, respectively,(Lu_(0.94)Gd_(0.03)Ce_(0.03))₃Sr_(0.34)Al₅O₁₂F_(0.68),(Lu_(0.90)Gd_(0.07)Ce_(0.03))₃Sr_(0.34)Al₅O₁₂F_(0.68), and(Lu_(0.84)Gd_(0.13)Ce_(0.03))₃Sr_(0.34)Al₅O₁₂F_(0.68).

It is noted that the Sr series emitted at a higher relativephotoluminescent intensity when compared to the Ba series, but oneskilled in the art will know to draw conclusions carefully, as severalother variables were changed at the same time (e.g., Gd content,alkaline earth amounts, and halogen concentrations).

Shown in FIGS. 16A-B are the x-ray diffraction patterns of both the Baseries and the Sr series of phosphors whose luminosity data was depictedin FIGS. 17A-B.

Although the present embodiments and their advantages have beendescribed in detail, it should be understood that various changes,substitutions, and alterations may be made without departing from thespirit and scope of the embodiments as defined by the appended claims.

TABLE 1 Testing results of Lu_(2.91)Ce_(0.09)Al₅O₁₂ with different MgF₂levels of additive Emission Peak Relative Particle MgF₂ WavelengthIntensity Size (wt %) CIE x CIE y (nm) (%) D50 (um) 1 0.3635 0.5862526.88 58.04 4.01 2 0.3554 0.5778 529.56 78.47 7.38 3 0.3336 0.5776514.22 105.13 9.30

TABLE 2 Testing results of Lu_(2.91)Ce_(0.09)Al₅O₁₂ with differentlevels of SrF₂ additive Emission Peak Relative Particle SrF₂ WavelengthIntensity Size (wt %) CIE x CIE y (nm) (%) D50 (um) 1 0.3677 0.5732534.64 71.65 3.84 2 0.3677 0.5732 534.64 85.82 5.24 3 0.3555 05718532.43 112.40 9.90 5 0.3405 0.5748 523.44 107.67 11.38

1. A cerium-activated, yellow-green to yellow-emitting aluminatephosphor comprising the rare earth lutetium, at least one alkaline earthmetal, aluminum, oxygen, at least one halogen, and at least one rareearth element other than lutetium, wherein the phosphor is configured toabsorb excitation radiation having a wavelength ranging from about 380nm to about 480 nm, and to emit light having a peak emission wavelengthranging from about 550 nm to about 600 nm.
 2. The yellow-green toyellow-emitting aluminate phosphor of claim 1, wherein the excitationradiation has a wavelength ranging from about 420 nm to about 480 nm. 3.The yellow-green to yellow-emitting aluminate phosphor of claim 1,further including any of the rare earth elements scandium (Sc),gadolinium (Gd), lanthanum (Lm), and terbium (Tb).
 4. A yellow-green toyellow-emitting aluminate phosphor comprising a halogenated aluminate,an M²⁺X₂ additive, and a cerium activator; where M²⁺is a divalent,alkaline earth metal selected from the group consisting of Mg, Sr, Ca,and Ba; X is a halogen selected from the group consisting of F, Cl, Br,and I; and wherein the M²⁺X₂ additive is included in the phosphor inamounts ranging up to about 5 wt %, endpoint inclusive.
 5. Theyellow-green to yellow-emitting aluminate phosphor of claim 4, whereinthe phosphor comprises an aluminate configured to absorb excitationradiation having a wavelength ranging from about 420 nm to about 480 nm,and to emit light having a peak emission wavelength ranging from about550 nm to about 600 nm.
 6. The yellow-green to yellow-emitting aluminatephosphor of claim 4, wherein the M²⁺X₂ additive is at least one of MgF₂,SrF₂, or BaF₂.
 7. The yellow-green to yellow-emitting aluminate phosphorof claim 4, wherein the aluminate is a LAG-based compound having theformula (Lu,Ce)₃Al₅O₁₂.
 8. The yellow-green to yellow-emitting aluminatephosphor of claim 4, further including any of the rare earth elementsscandium (Sc), gadolinium (Gd), lanthanum (La), and terbium (Tb).
 9. Ayellow-green to yellow-emitting halogenated aluminate phosphor havingthe formula: (Lu_(1-x-y)A_(x)Ce_(y))₃B_(z)Al₅O₁₂C_(2z); where A is atleast one of Sc, La, Gd, and Tb; B is at least one of Mg, Sr, Ca, andBa; C is at least one of F, C, Br, and I; 0≦x≦0.5; 001≦y≦0.2; and0.001≦z≦0.5.
 10. The yellow-green to yellow-emitting halogenatedaluminate phosphor of claim 9, wherein A is Gd.
 11. The yellow-green toyellow-emitting halogenated aluminate phosphor of claim 9, wherein B isBa or Sr.
 12. The yellow-green to yellow-emitting halogenated aluminatephosphor of claim 9, wherein C is F.
 13. A white LED comprising: aradiation source for configured to provide radiation having a wavelengthgreater than about 280 nm; a cerium-activated, yellow-green toyellow-emitting aluminate phosphor comprising the rare earth lutetium,at least one alkaline earth metal, aluminum, oxygen, at least onehalogen, and at least one rare earth element other than lutetium,wherein the phosphor is configured to absorb excitation radiation havinga wavelength ranging from about 380 nm to about 480 nm, and to emitlight having a peak emission wavelength ranging from about 550 nm toabout 600 nm; and at least one of a red-emitting phosphor or ayellow-emitting phosphor.
 14. The white LED of claim 13, wherein thered-emitting phosphor is a nitride.
 15. The white LED of claim 14,wherein the nitride has the formula (Ca,Sr)AlSiN₃:Eu²⁺.
 16. The whiteLED of claim 13, wherein the yellow-emitting phosphor is a silicate. 17.The white LED of claim 16, wherein the silicate has the formula(Sr,Ba)₂SiO₄:Eu²⁺.